1. Field of the Invention
This invention relates to a method for mass production of single-crystal epitaxial layers of semiconductors from the liquid phase. More particularly, this invention is adaptable for growing the epitaxial layers with single-layer structure, double-layer structure or multi-layer structure of Group III-V semiconductors such as GaP and Ga.sub.1-x Al.sub.x As (0 &lt; x &lt; 1).
The vapor phase epitaxial growth method for manufacturing epitaxial layers of GaAs and GaAs.sub.1-x P.sub.x (0 &lt; x &lt; 1) on a GaAs substrate, or the epitaxial layers of GaAs.sub.1-x P.sub.x (0 &lt; x &lt; 1) on the GaP substrate has made rapid progress. Recently, 30 - 50 wafers, with each having a surface area of 10 - 20 cm.sup.2, have been able to be manufactured in a single epitaxial growth run by the method. Contrary to this method, the liquid phase epitaxial growth method for growing the epitaxial layers of GaP on the GaP substrate and Ga.sub.1-x Al.sub.x As (0 &lt; x &lt; 1) on the GaAs substrate has been often pointed out to be lacking in mass producibility, while further pointing out that the method can provide very high quality epitaxial layers for light emitting diodes and semiconductor lasers.
2. Description of the Prior Art
Various attempts at imparting mass productivity to the liquid phase epitaxial growth method are dealt with in detail in, for example, a paper by R. H. Saul and O. G. Lorimor, entitled "Liquid Phase Epitaxy Processes for GaP LED's", Journal of Crystal Growth, Vol. 27, (1974), pp. 183 - 192. Especially, the thin-melt multi-slice slider method (hereinafter to be referred to as the thin-melt method) is introduced therein in detail. This method has advantages in that it enables one to obtain an epitaxial growth layer of a uniform thickness having an especially smooth surface, that the quantity of solvent for the liquid solution required by it is small, that it enables one to obtain an epitaxial growth layer of a single layer structure or multi-layer structure (particularly, pn junction layer) by a single process, and that the GaP light emitting diodes obtained by it have an excellent quantum efficiency. FIGS. 1(a) and (b) are diagrams explanatory of the thin-melt method. As shown in FIG. 1(a), a solution reservoir 1 containing the solution 2 and wells 5, as well as a slider 6 which also serves as a supporting base for the substrate 3, a cover plate 4 and a cover plate 9 which has vapor-pervious thin holes 7 are all provided in a long horizontal reaction tube which is heated to a uniform temperature. The substrate 3 is a single crystal of GaP and the liquid solution contains GaP of a concentration that saturates the solvent Ga.
Sulphur S for n- type impurity, nitrogen N for radiative center and zinc Zn for p-type impurity are doped into the growth layers respectively from H.sub.2 S gas, NH.sub.3 gas and Zn vapor contained in the hydrogen or inert or neutral gas. The slider 6 is moved to the right from the state shown in FIG. 1(a), and the liquid solution 2 is supplied into the well 5 as shown in FIG. 1(b) to prepare a supplied liquid solution that is the thin melt 8 of a small portion of liquid solution, which is slowly cooled in that position with the flow of gas. The gas contains H.sub.2 S and NH.sub.3 at the beginning so as to grow an n layer containing N and S. Then the cooling is stopped and the gas is changed to a gas containing Zn and NH.sub.3 but no H.sub.2 S, and cooling is continued. In this way an n layer and a p layer are grown in continuation on an n.sup.+ substrate in a single process. This method has a number of advantages as already mentioned, but it has a shortcoming in that it necessarily calls for a large idle space because of the operation of a horizontally long slider 6. From FIG. 1, it will be obvious that the space required for the actual growth of epitaxial layers on a substrate 3 is found to be too small for the whole space occupied by the apparatus.
On the other hand, a method in which a rotatable circular plate is used, was proposed in one of the attempts at improving the mass producibility of the liquid phase epitaxial growth method. One example of the embodiment of this method is shown in FIG. 2. FIG. 2(b) is a top view and FIG. 2(a) is a cross-sectional view taken along B-A-C of FIG. 2(b).
The liquid solution 2 is put in the solution reservoir 1, which is placed on the substrate holder 10 of a circular plate. The substrate holder 10 has a well 5 and the substrate 3 is placed therein.
The solution reservoir 1 and the substrate holder 10 are so made that they are mutually rotatable about the central axis A-A'. If they are mutually rotated 90 degrees from the positions shown in FIG. 2(b), then the substrate 3 positions just under the liquid solution 2 and the epitaxial growth can be accomplished in that position.
This method (which is called the rotating slider method) has an advantage in that, unlike the method illustrated in FIG. 1, it does not call for a horizontally long slider. Nevertheless, it still has a shortcoming in that the space actually taken or usable in the structure for growing epitaxial layers on the substrate 3 is too small.